1. Field of the Invention
The present invention relates to a wavelength stabilizing unit and a laser module using the same wavelength stabilizing unit and, in particular, the present invention relates to a wavelength stabilizing unit capable of stabilizing a wavelength of emitted laser light with high accuracy, simplifying a structure thereof and reducing a size thereof and a wavelength stabilized laser module using the same wavelength stabilizing unit.
2. Description of the Prior Art
A semiconductor laser has been used as a light source of an optical fiber communication system. In particular, a single axial mode semiconductor laser such as DFB (distributed feedback) laser has been employed for optical fiber communication over distances of tens of kilometers or more in order to restrict wavelength dispersion. However, though the DFB laser oscillates at a single wavelength, its oscillation wavelength is changed depending on the temperature of the semiconductor laser device and/or an injected current. Moreover, in the optical fiber communication system, in which it is important to keep strength of light output of a semiconductor laser light source at a constant level, control is conventionally exercised so as to keep the temperature of the semiconductor laser device and the output light strength of the semiconductor laser light source in constant levels, respectively. Basically, by keeping the temperature of the semiconductor laser device and the injected current at constant levels, light output and oscillation wavelength of the semiconductor laser device is maintained constant. However, if the quality of semiconductor laser device is degraded due to longtime use, the injected current required for keeping the light output at constant level increases, causing the oscillation wavelength to change. However, since an amount of the change of the oscillation wavelength is slight, substantially no problem occurs in the conventional optical fiber communication system.
In recent years, a dense wavelength division multiplexing (DWDM) method in which multiple pieces of light each having a different wavelength are multiplexed in one optical fiber becomes mainstream in the conventional optical fiber communication system and the interval among a plurality of oscillation wavelengths used for the DWDM system becomes as narrow as 100 GHz or 50 GHz. In this case, the degree of wavelength stabilization required for the semiconductor laser device, which is used as the light source, is, for example, wavelength change within xc2x11 nm for a 2xc3x97105 hours (about 25 years) use. Therefore, the conventional wavelength stabilization by using the conventional constant element temperature-constant light output control becomes not sufficient to obtain the required degree of wavelength stabilization. Moreover, even if the temperature of the semiconductor laser itself is successfully controlled so as to remain constant, the oscillation wavelength is changed slightly when the ambient temperature around the semiconductor laser device changes and an amount of such slight change in the oscillation wavelength may become a problem in the recent conventional optical fiber communication system.
In order to restrict such change in the oscillation wavelength of the semiconductor laser light to thereby stabilize the oscillation wavelength, some wavelength stabilized devices have been proposed in such as JP H10-209546 A (Japanese Patent No. 2989775), JP H4-157780 A (Japanese Patent No. 2914748), JP H9-219554 A, JP H10-79723 and JP H9-121070. However, each of the proposed wavelength stabilized devices requires a large number of parts and a large space, so that it becomes difficult to house the wavelength stabilized device in the generally used casing of the conventional semiconductor laser module. Moreover, the setting of a reference wavelength to be stabilized is difficult and the fabrication cost becomes high.
On the other hand, Japanese Patent Application No. 2000-67606, which corresponds to U.S. patent application Ser. No. 09/804,499 assigned to the assignee of the present application and will be referred to as xe2x80x9cprior applicationxe2x80x9d, hereinafter, proposes a wavelength stabilized laser module capable of solving the above problems. FIG. 10A shows an example of a construction of a wavelength stabilized laser module proposed in the prior application and FIG. 10B shows a portion thereof in an enlarged scale. The wavelength stabilized laser module shown in FIG. 10A and FIG. 10B includes a semiconductor laser 801 housed in a casing 809, a lens 802 for converting rearward diverging light emitted from the semiconductor laser into parallel light bundle, a first photoelectric conversion element 805, which directly receives a portion of the parallel light bundle passed through the lens 802 and converts the light portion into an electric signal, an etalon-type filter 831, which receives another portion of the parallel light bundle passed through the lens 802, and a second photoelectric conversion element 806, which converts light passed through the etalon-type filter 831 into an electric signal.
The semiconductor laser 801 is mounted on a substrate 807 equipped with a Peltier element so that temperature thereof during operation can be regulated. An incident angle of light to the etalon type filter 831 can be regulated by an angle regulating mechanism, which is not shown. The first photoelectric conversion element 805 and the second photoelectric conversion element 806 are arranged in parallel on a supporting substrate 849 to form an array type optical detector 804. The optical detector 804 is slanted with respect to an optical axis of the incident light in order to prevent light from being reflected back to the semiconductor laser.
The wavelength stabilized laser module constructed as mentioned above has is highly accurate, has small number of parts, has good space efficiency and has a size small enough to be housed in a casing of the semiconductor laser module, which has been used usually. Further, since the assembling of the wavelength stabilized laser module and the positional regulation thereof are easy, the fabrication cost thereof can be substantially reduced.
As mentioned above, it has been found that the considerable effects can be obtained by the wavelength stabilized laser module proposed in the prior art application. However, the inventors of the present invention conducted various experiments on the proposed wavelength stabilized laser module and have found that the latter wavelength stabilized laser module has some points to be improved.
The points to be improved will be described in detail with reference to FIG. 12, which shows graphs illustrating relations between an oscillation wavelength xcex of a semiconductor laser in abscissa and currents Im of a strength monitoring PD (photo diode) when laser light emitted from the semiconductor laser is incident directly on the photoelectric conversion element and a wavelength monitoring PD when the output light is incident on the photoelectric conversion element after passed through a predetermined filter such as an etalon-type filter, in ordinate. Further, as described in the prior art application, the oscillation wavelength of the semiconductor laser is changed depending on not only change of temperature of the photoelectric conversion element but also change of injected current of the semiconductor laser, as shown in FIG. 13A and FIG. 13B. On the contrary, the light output is changed by not only change of the injected current but also change of temperature as shown in the same figures. Considering a case where the oscillation wavelength of the semiconductor laser is controlled to a reference wavelength xcexo while keeping the optical output thereof constant, on the basis of the graph shown in FIG. 12, it is possible to simultaneously control the oscillation wavelength and the output light of the semiconductor laser by controlling the current Ipd1 detected by the strength monitoring PD and the current Ipd2 detected by the wavelength monitoring PD such that (Ipd1+Ipd2) and (Ipd1xe2x88x92Ipd2) become constant, respectively. This control can be realized by controlling at least either one of the injected current and the temperature of the semiconductor laser as will be clear from FIG. 12.
However, according to various experiments conducted by the present inventors, it has been found that the stability of the oscillation wavelength of the wavelength stabilized laser module shown in FIG. 10 can be further improved. That is, in the wavelength stabilized laser module shown in FIG. 10, the current of the first photoelectric conversion element 805, that is, the strength monitoring PD, exhibited unstable change depending on wavelength as shown by portions Q1 to Q4. From this, it has been found that, if the wavelength dependency of the current of the first photoelectric conversion element (PD) can be made closer to the wavelength dependency of the current of the strength monitoring PD shown by the solid line in FIG. 12, the oscillation wavelength control can be performed highly accurately. On the basis of this knowledge, the cause of occurrence of the unstable portions Q1 to Q4 was investigated and it has been found that the unstable wavelength dependency of the current of the strength monitoring PD such as shown by Q1 to Q4 in FIG. 12 occurs due to stray light incident on the first photoelectric conversion element 805, which contains reflection light 850, which is a portion of the parallel light bundle incident on a side face 833 of the filter 831 and reflected by the side face 833 to the photoelectric conversion element 805, and multiplexed reflection light 852, which is a light incident on the side face 833 and reflected to the photoelectric conversion element 805 after reflected a plurality of times within the filter 831.
The present invention was made to solve the above mentioned problems and has an object to provide a low cost wavelength stabilizing unit capable of further stabilizing an oscillation wavelength of a semiconductor laser with higher accuracy by stabilizing current of a photoelectric conversion element for monitoring strength of laser light by restricting stray light incident thereon, of reducing size thereof to a size small enough to be encased in a casing of a conventional semiconductor laser module, of reducing the number of parts and of very easily and highly accurately setting a reference wavelength at which the oscillation wavelength is to be stabilized, and a wavelength stabilized laser module using the same wavelength stabilizing unit.
In order to achieve the above object, a wavelength stabilizing unit according to the present invention comprises at least a first photoelectric conversion means converting a portion of a laser light emitted from a predetermined emitting point of a semiconductor laser equipped with a temperature regulation means into an electric signal, a wavelength filter having transmittance continuously changed depending on wavelength of a portion of the emitted light incident directly thereon, a second photoelectric conversion means for converting light passed through the wavelength filter into an electric signal and stray light blocking means for preventing the emitted light incident on a side face portion of the wavelength filter from being incident on the first photoelectric conversion means through the wavelength filter, wherein the electric signals from the first and second photoelectric conversion means are operated to obtain a control signal for stabilizing wavelength of the emitted light and the control signal is fedback to at least one of the semiconductor laser and the temperature regulating means thereof to stably output a laser light having a reference wavelength at which the oscillation wavelength of the semiconductor laser is stabilized.
This wavelength stabilizing unit may further comprise light bundle paralleling means for converting the divergent light emitted from the semiconductor laser into a parallel light bundle, wherein the first photoelectric conversion means receives a portion of the parallel light bundle directly. The wavelength filter may receive another portion of the parallel light bundle directly and the light bundle paralleling means may be an optical lens.
It is preferable that the parallelism of the parallel light bundle, that is, angle of deviation with respect to the optical axis, is within xc2x12xc2x0.
With this scheme, adverse effect on the transmittance due to that the incident angle differs depending on location of the wavelength filter is minimized, so that the wavelength stabilization can be achieved with very high accuracy.
The stray light blocking means may be a stray light blocking wavelength filter fabricated such that a straight line connecting the emitting point and an edge portion of an incident surface of the wavelength filter on which the emitted light is incident and an extension of the straight line do not cross the side face of the wavelength filter.
In the stray light blocking means in a case where the wavelength stabilizing unit includes the light bundle paralleling means, the wavelength filter may be mounted such that a distance between a first edge portion of the incident surface thereof closest to the optical axis of the parallel light bundle and the light bundle paralleling means is larger than a distance between a second edge portion of the incident surface remotest from the optical axis of the parallel light bundle and the light bundle paralleling means and the wavelength filter does not cross the optical axis of the parallel light bundle. Alternatively, a stray light blocking wavelength filter fabricated by machining the side faces of the wavelength filter such that it is not irradiated with the parallel light bundle may be used. In such case, the stray light blocking wavelength filter may be provided by machining the wavelength filter such that at least the side faces thereof, which are irradiated with the parallel light bundle, do not cross a line parallel to the optical axis of the parallel light bundle passing the edge portion of the incident surface of the wavelength filter on which the parallel light bundle is incident. Alternatively, an angle of the side face of the stray light blocking wavelength filter with respect to the incident surface thereof on which the parallel light bundle is incident may be set smaller than an angle of the incident surface thereof with respect to a portion of the optical axis of the parallel light bundle, which is on the side of the first photoelectric conversion means from the stray light blocking wavelength filter.
Alternatively, the stray light blocking means may be a stray light blocking wavelength filter having the side faces thereof coated with a predetermined material selected from a group containing at least non-reflection film materials and light absorbing film materials or may be a stray light blocking wavelength filter having side faces coated with a reflection film such that the first photoelectric conversion element is not irradiated with light reflected by the side faces. Alternatively, it may be possible to form the stray light blocking wavelength filter by making the side faces irregular by roughing it.
Preferably, the wavelength filter has transmission characteristics in which the transmittance monotonically increases or decreases depending on wavelength in a wavelength band containing the reference wavelength at which the oscillation wavelength of the semiconductor laser is to be stabilized.
By selecting a wavelength filter whose change of transmittance with change of wavelength within the wavelength band including the reference wavelength, that is, gradient of transmission spectrum, is monotonically increases or decreases, the second photoelectric conversion means can immediately detect wavelength of the laser light, which varies on a short wavelength side or a long wavelength side with respect to the reference wavelength, as a change of strength of the light transmitted through the filter on a dark side or a bright side.
It is preferable that the wavelength filter can change the gradient of the wavelength dependent change of transmittance by regulating the incident angle of the incident light.
If the gradient of wavelength-dependent transmittance can be changed by regulating the incident angle of light, it becomes possible to improve the sensitivity of detection related to wavelength variation to thereby stabilize the wavelength with high accuracy by making the gradient sharp and, on the other hand, to expand the band width of wavelength capable of detecting the variation by making the gradient gentle.
It is preferable that the wavelength filter has a single peak transmission characteristics in which the transmittance in a wavelength band, which does not include the reference wavelength, becomes maximum or minimum. If the reference wavelength is in the maximum transmission band or the minimum transmission band of the transmission characteristics of the wavelength filter, the sensitivity for wavelength variation is substantially lowered. When the transmission characteristics of the wavelength filter is of single peak type, it becomes possible to perform a highly accurate wavelength detection in a wide wavelength band within the wavelength band in which the semiconductor laser can oscillate, except very narrow maximum or minimum transmission band if any.
The wavelength filter may use a multi-layered filter fabricated by forming a multi-layered film-by-film layers having different dielectric materials on a transparent substrate.
Since thickness of the transparent substrate such as a glass substrate of the multi-layered filter can be set arbitrarily, the filter can be made compact by reducing the thickness of the glass substrate.
Alternatively, an etalon type filter having a transmittance period in which the transmittance is periodically changed between extreme value and minimal value with constant wavelength interval may be used as the wavelength filter.
Since the etalon type filter has a plurality of extreme points and minimal points within the wavelength band, which the semiconductor laser can oscillate, it is possible to set the reference wavelengths in spectrum gradients each defined by a line connecting paired extreme point and minimal point, so that a stabilization of a plurality of reference wavelengths becomes possible in a multiplexed light transmission system, which utilizes a wavelength variable semiconductor laser as a light source, by using a single wavelength stabilizing unit.
It is preferable that the semiconductor laser is of the wavelength variable type capable of oscillating at a plurality of wavelengths depending on temperature and the wavelength interval of the transmittance period of the etalon type filter is set according to the following equation:
D=(1xe2x88x92Tetalon/TLD)xc3x97D0xe2x80x83xe2x80x83(1)
where D represents the wavelength interval of transmittance period of the etalon type filter, D0 represents an interval between a plurality of oscillation wavelengths of the semiconductor laser, Tetalon represents an amount of change of a center frequency when temperature of the etalon type filter is changed by 1xc2x0 C. and TLD represents an amount of change of oscillation frequency of the semiconductor laser when temperature thereof is changed by 1xc2x0 C. Incidentally, the center wavelength represents a certain one of the wavelengths, at which the transmittance becomes maximum.
By using a semiconductor laser having temperature dependent wavelength and setting the wavelength interval of the transmittance period of the etalon type filter according to the Equation 1, it becomes possible to set a plurality of reference wavelengths used in the multiplexed light transmission system in the spectrum gradients connecting the extreme points and the minimal points of the transmittance period. As a result, the plurality of the wavelengths at which the semiconductor laser oscillates can be stabilized by using a single wavelength stabilizing device.
The etalon-type filter is preferably formed of a transparent material having refractive index larger than that of quartz glass. It is preferable that the transparent material having refractive index larger than that of quartz glass is Si (silicon)-based material.
By using a transparent material, which has refractive index larger than that of quartz glass and is used as a basic material of, for example, an etalon type filter or a multi-layered filter, it is possible to further reduce the thickness of the wavelength filter to thereby further reduce a space required for the device. Since silicon is transparent, has refractive index larger than that of quartz and is a relatively inexpensive material widely used in the semiconductor field, it is the most preferable material of the wavelength filter used in the present invention.
It is preferable that the first photoelectric conversion means and the second photoelectric conversion means are arranged in parallel on a supporting substrate to form an array type optical detector.
Since, in the wavelength stabilizing unit of the present invention, there is no need of complicated angle regulations of the first photoelectric conversion means and the second photoelectric conversion means, the number of parts and the number of assembling steps are reduced by using the optical detector formed by arranging them in parallel on one and the same supporting substrate, so that the fabrication cost can be reduced.
It is preferable that a light receiving surface of the first photoelectric conversion means is slanted with respect to the optical axis of the incident light.
With such arrangement of the first photoelectric conversion means with respect to the optical axis of the incident light, reflection of light from the light receiving surface of the first photoelectric conversion means back to the semiconductor laser is excluded, so that the change of oscillation characteristics of the semiconductor laser due to returning of light can be restricted.
The wavelength stabilized laser module according to the present invention may include a semiconductor laser, temperature regulating means for regulating temperature of the semiconductor laser and a wavelength stabilizing unit for stabilizing oscillation wavelength of the semiconductor laser. Any of the wavelength stabilizing units previously described can be used as the wavelength stabilizing unit of the wavelength stabilized laser module.
In this case, the semiconductor laser may be constructed by integrating it with an exciton absorption type semiconductor optical modulator.
When the semiconductor laser is integrated with the exciton absorption type semiconductor optical modulator, it is possible to construct the whole optical transmission system compact compared with a usual case where a DFB laser and an external modulator are constructed as a separate module.
The temperature regulating means is preferably a Peltier element. Since the Peltier element can precisely set a temperature in an arbitrary temperature range by electronic control and has a thin wall structure, it can be housed in the casing of the wavelength stabilized laser module by sticking it to a substrate of the wavelength stabilized laser module.
It is preferable that the wavelength stabilized laser module according to the present invention includes an optical fiber as laser light output means and at least the semiconductor laser, the temperature regulating means and any of the wavelength stabilizing unit are encased in a single casing.
The above mentioned wavelength stabilizing unit and the wavelength stabilized laser module using the wavelength stabilizing unit, according to the present invention, are constructed with a minimum number of parts and can be regulated easily. Therefore, it is possible to easily incorporate the present wavelength stabilized laser module even in a small casing for a conventional semiconductor laser module having no wavelength stabilizing unit.